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fertilized egg (embryo) 1st instar larva 2nd instar larva 3rd instar larva Pupa ( metamorphosis ) 1 day 2 days 4 days adult ™ ¢ (reproductive in ~2 days) XX XY If a gene functions at different times to do different things, temperature shifts of ts mutants can reveal those various functions Drosophila life

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determination of the “temperature-sensitive period” (TSP) by single temperature shifts up or down point (time) in development when temp. shifted embryo larva pupa adult ultimate phenotype all OK all mut restrictive unshifted permissive unshifted shift rstrctv to prmsv shift prmsv to restrv TSP start : when begins drop TSP end : when wildtype TSP single

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determination of the “temperature-sensitive period” (TSP) point (time) in development when exposed to rstrctv. temp for some fixed period of time (relatively short for high resolution). embryo larva pupa adult ultimate phenotype all OK all mut restrictive unshifted permissive unshifted TSP Temperature pulses (double-shifts) are more definative: shift from prmsv to restrv AND BACK P P R gene function needed: 12

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Temperature-sensitive mutant alleles help us recognize (“dissect”) a gene’s pleiotropy (multiple, “unrelated” functions) point (time) in development when exposed to the restrictive temp. embryo larva pupa adult ultimate phenotype all OK all mut restrictive unshifted permissive unshifted two TSPs: time of function 1 2 Consider if: needed for an adult behavior essential for development & viability vs.behavior viability low behavior abnormal mutant animal survives, allowing us to see the behavioral abnormality

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(1) What kinds can we make? (categories) (2) How do we make them? (mutagenesis) (3) How do we find them? (mutant screens & selections) Mutations (changes in DNA): the lifeblood of genetic analysis (4) Why bother?

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(1) …generally easier to induce germline mutations in males than in females. in sperm: little to “distract” the mutagen from DNA With chemical mutagens:

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(wt) after 1st zygotic division subsequent rounds of replication do not invariably cause errors: With chemical mutagens: (2) …often progeny from mutated sperm are genetically mosaic for new mutations only one of the two complementary bases in sperm are modified: GC -> G * C -> G * T & GC in sperm G * T -> G * C & AT wt mutant Question: if an animal is mosaic for the new mutant base pair, under what (developmental) circumstances will that new mutation not be transmitted to the progeny? Answer: if the mutant base pair doen't end up in germline stem cells.

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Radiation (the first experimental mutagen discovered) ionizing (higher energy) X-rays,  -rays, cosmic rays generates free radicals:chemical bull in the china shop …can induce double-strand DNA breaks important to repair at any cost!! …but what repair template available? -- information on the sister chromatid, but only after DNA replication -- information on the homologous chromosome in diploid organisms If none can be found (or found in time), just stick the DNA together blindly. Hence,ionizing radiation causes just about every molecular category of genetic change imaginable. (double helix shown, not sister chromatids)

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Reading: “Transposable genetic elements move from place to place in the genome” pp (Chp 14) Also: p548 (insertion sequences in bacteria) A practice problem set dealing with mutant categories is available on the website. Answers will be posted next Monday. Lecture schedule: Wed 3/14: genetic screens Fri 3/16: sensitized screens Mon 3/19: genetic mosaics in screens (hence: no RNAi and one less sex lecture)

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ionizing (higher energy) X-rays,  -rays, cosmic rays generates free radicals:chemical bull in the china shop …among other damage, can induce double-strand DNA breaks cell must repair this damage at any cost to avoid dying after next cell division!! …but what repair template available? If none can be found (or found in time), just stick the DNA together blindly. Hence,ionizing radiation causes just about every genetic change imaginable. (double helix, not sister chromatids)